This invention relates in general to refining of molten metal, and more particularly, to a method for removing dissolved gases and non-metallic impurities from molten metal without the emission of corrosive or environmentally harmful gases and fumes.
Molten metal, prior to casting, contains many impurities which, if not removed, cause high scrap loss in casting, or otherwise cause poor metal quality in products fabricated therefrom. The principal objectionable impurities are dissolved gases and suspended non-metallic particles such as metal oxides and refractory particles.
It is an object of this invention to provide a process for refining metal with refining gas which efficiently removes dissolved gases and other non-metallic impurities from the metal in a continuous process at high metal throughput rates.
The above object and others, which will be apparent to those skilled in the art, are achieved by a process for removing dissolved gases and non-metallic impurities from a molten metal selected from the group consisting of magnesium, copper, zinc, tin and lead comprising the steps of:
1. FEEDING THE MOLTEN METAL INTO A REFINING ZONE;
2. MAINTAINING A PROTECTIVE ATMOSPHERE ABOVE THE SURFACE OF THE MOLTEN METAL AT A POSITIVE PRESSURE RELATIVE TO THE AMBIENT PRESSURE, THEREBY PREVENTING INFUSION OF AIR AND MOISTURE INTO SAID ZONE AND CONTACT OF THE MOLTEN METAL THEREWITH;
3. INTRODUCING A REFINING GAS IN THE FORM OF DISCRETE BUBBLES INTO THE MOLTEN METAL BENEATH THE SURFACE OF THE MELT;
4. STIRRING THE MOLTEN METAL IN THE REFINING ZONE TO CREATE A CIRCULATION PATTERN IN THE MOLTEN METAL RELATIVE TO THE POINTS OF ENTRY OF THE GAS BUBBLES IN THE MELT SUCH THAT THE GAS BUBBLES INTRODUCED INTO THE MELT ARE TRANSPORTED SUBSTANTIALLY RADIALLY OUTWARD, RELATIVE TO SAID POINTS OF ENTRY OF THE BUBBLES, THEREBY PROLONGING THE RESIDENCE TIME OF THE GAS BUBBLES IN THE MELT AND CAUSING THE GAS BUBBLES TO COME INTO INTIMATE CONTENT WITH SUBSTANTIALLY THE ENTIRE MASS OF MOLTEN METAL IN SAID REFINING ZONE;
5. WITHDRAWING THE SPENT REFINING GAS AND THE DISSOLVED GASES RELEASED BY THE METAL WHILE COLLECTING AND SEPARATING THE OTHER NON-METALLIC IMPURITIES IN A SLAG LAYER ON THE SURFACE OF THE MOLTEN METAL; AND
6. WITHDRAWING THE REFINED MOLTEN METAL FROM SAID REFINING ZONE.
The term "refining gas" as used herein is meant to include gases which are conventionally used in the refining of magnesium, copper, zinc, tin, and lead. The common characteristic of these refining gases is that they are inert towards the molten metal being refined. Argon and nitrogen or mixtures thereof are preferred although other inert gases of the periodic table are suitable for the present invention. Other useful refining gases are hydrogen and carbon monoxide or mixtures thereof with each other or the inert gases of the periodic table. It will be noted that hydrogen and carbon monoxide may be used in instances where they will not react with the molten metal, but will react with gaseous impurities such as oxygen. Other reactive gases with similar characteristics can also be used such as sulfur hexafluoride, chlorine, and halogenated hydrocarbons. Selection of a particular refining gas is generally made in accordance with the characteristics of the particular metal being refined.
The term "metal" as used throughout the specification and claims is meant to include pure metal as well as alloys of the metal.